1. Field of the Invention
The present invention relates to the field of transistor design. More particularly, it is related to the production of heterojunction bipolar transistors.
2. Background Information
The operating principles of heterojunction bipolar transistors, as well as their main fields of application, are well known to those skilled in the art. These operating principles and fields of application have been the object of much literature. Reference can be made, for example, to the following papers: 1) "Heterostructure Bipolar Transistors and Integrated Circuits", H. Kroemer, Proc. IEEE 25 (1), January 1982; 2) "GaAlAs/GaAs Heterojunction Bipolar Transistors: Issues and Prospects for Applications" P. Asbeck, M. Chang, J. Higgins, N. Sheng, G. Sullivan, K. Wang, IEEE Trans. Electron. Dev. 36 (10), October 1989; 3) "GaAs Heterojunction Bipolar Transistor Device and IC Fabrication Technology for High Performance Analog and Microwave Applications", M. Kim, A. Oki, G. Gorman, D. Umemoto, J. Camou, Trans. Microwave Theory Tech., 37 (9), September 1989.
As shown in the accompanying FIG. 1, conventional heterojunction bipolar transistor structures generally comprise a semi-insulating substrate 10, a GaAs n+ collector contact layer 12, a GaAs n- collector layer 14, a GaAs p+ layer forming the base 16, a GaAlAs n emitter layer 18 and a GaAs n+ emitter contact layer 20.
As shown in the accompanying FIG. 1, the technology most commonly employed for treating these structures is double mesa technology. This technology consists in effecting a first etch at 22 to contact the base layer 16, and then a second etch at 24 to contact the collector contact layer 12. Ohmic contacts 30, 32 and 34 are then deposited on the emitter contact layer 20, base layer 16 and collector contact layer 12.
In this type of technology, the emitter 18--base 16 junction is exposed by etching 22 which induces high surface currents.
To improve the component's performance, emitter 30 and base contacts 32, and also possibly collector contacts 34, must be self-aligned, and the size of the component also reduced.
However, reducing the size of the component increases the edge-to-surface area ratio of the emitter 18--base 16 junction, and makes the surface recombination problem of the emitter 18--base 16 junction even more critical.
Surface currents result in a very significant drop in gain which prevents the component from being used under the best conditions.
Many solutions have already been suggested in an attempt to overcome this problem.
For example, in the papers "Near-ideal Transport in an AlGaAs/GaAs Heterostructure Bipolar Transistor by Na2S-9H2O Regrowth", R. Nottenburgh, C. Sandorff, D. Humphrey, T. Hollenbeck, R. Bhat, Appl. Phys. Lett., 52(3), January 1988 and "Suppression of the Emitter Size Effect on the Current Gain of AlGaAs/GaAs HBT by Utilizing (NH4)2SX Treatment", S. Shokata, H. Okada, H. Hayashi, Inst. Phys. Ser., 112, 1991, it was shown that surface recombinations could be reduced by exposing the surface to solutions of the type Na.sub.2 S.9H.sub.2 O and (NH.sub.4)2S.sub.x. This passivation, however, poses real technological problems: stability in time, compatibility with future technological treatments, etc.
The paper "Super-gain AlGaAs/GaAs Heterojunction Bipolar Transistors Using an Emitter Edge-thinning Design", H. Lin, S. Lee, Appl. Phys. Lett., 47(8), 1985, suggested conserving part of the GaAlAs emitter at the surface of the base, and removing it from only beneath the base ohmic contacts. The drawback of this technique is that, first, it is impossible to control the thickness of the conserved layer, and secondly, that this GaAlAs layer must be cleared in order to produce an ohmic contact of low resistivity. This technique is therefore difficult to make use of, particularly in the case of self-aligned technology.
The paper "Submicron Scaling of AlGAs/GaAs Self-aligned Thin Emitter Heterojunction Bipolar Transistors (SATE-HBT) With Current Gain Independent of Emitter Area", R. J. Malik, L. M. Lunardi, R. W. Ryan, S. C. Shunk, M. D. Feuer, Electron. Lett., 25(17), August 1989, suggested using an ultrathin GaAlAs emitter layer (15 nm) and contacting the base with an AuBe diffusing contact. With this technique, the ohmic contact presents high resistivity. Moreover, the emitter layer thickness requirements result in an extremely large reduction in gain, making this structure virtually unusable. This technology has to the knowledge of the inventors since been abandoned.
Document EP-A-384 113 "Multilayer Base Heterojunction Bipolar Transistor" suggested using a graded Al base, or directly a base made up of 2 successive layers, the upper layer of which is a p type GaAlAs layer with a low Al percentage. This last structure poses a certain number of difficulties, particularly non-selective etching between 30- and 10-percent GaAlAs, higher resistivity contact on 10-percent GaAlAs than on GaAs, lower effectiveness of passivation as Al percentage is reduced, and reduction of injection efficiency.
Document EP-A-387 010 "Heterojunction Bipolar Transistor" also suggests local epitaxy of a high Al-percentage GaAlAs layer onto the base layer. While this technique overcomes selective etching problems, difficulties arise relating to the resistivity of the contact and restarting epitaxy.
Finally, the paper "High Speed Non-self-aligned GaInP/GaAs TEBT", P. Zwicknagl, U. Schaper, L. Scleicher, H. Siweris, K. Bachem, T. Lauterbach, W. Pletschen, Electron. Lett., 28 (3), 1992, suggested producing an emitter layer made up of 2 layers, one in GaAs and the other in GaInP. This solution uses selective etching and also passivation of the emitter-base junction by the GaInP layer. However, this structure, designated "Tunneling Emitter Bipolar Transistor" (TEBT) has a number of drawbacks; first, it presents low gain due to the use of a small energy gap material in the emitter, and secondly, the use of the CrAu contact suggested in this publication does not enable the base layer to be contacted through the GaInP. GaInP must therefore be removed from under the base contact. This technology is therefore difficult to adapt to small-size transistors and transistors made using self-aligned technology.
To conclude, all the solutions put forward to date present technological difficulties, particularly the problem of stability and reliability where several passivating single layers are deposited, non-selective etching when GaAlAs material is used to achieve passivation, the production of low resistivity p-type ohmic contacts on GaAlAs, low gain, etc.
The object of this invention is to perfect existing transistor structures by overcoming the drawbacks of the prior art.
More precisely, the object of this invention is, by using a very simple process, to produce transistors, particularly heterojunction bipolar transistors, of the highest possible performance, in particular high intrinsic gain by improved injection efficiency, this high gain being conserved on small-size transistors by the elimination of surface recombination currents.